The interactions which arise between the transition state and the enzymic structure during the act of catalysis are under investigation, in order to provide fundamental data for the electronic and geometric design of high-potency enzyme inhibitors which will resemble the transition state in its interaction with the enzyme. Such inhibitors may serve as drugs of extra-ordinary potency and specificity. The technique in use is the "proton inventory" method, according to which the effect of deuterium oxide on reaction rates is determined in successive experiments employing increasingly D-rich mixtures of light and heavy water. By simultaneously observing the rate effect of D, and its dependence on D concentration, one constructs a list of transition-state protons of altered binding state and an estimate of their actual binding state.